1. Field of the Invention
The present invention relates to an electronic ballast for the operation of at least one gas discharge lamp.
2. Description of the Related Art
By means of the alternating control of the switches of the inverter 2 there is generated on the output side of the inverter 2 a clocked, i.e. "chopped", high frequency a.c. voltage which serves as operating voltage for the gas discharge lamp. For the ignition of the gas discharge lamp 10, the output frequency of the inverter 2 is displaced into the vicinity of the resonance frequency of the series resonance circuit having the coil 4 and the capacitor 5, so that a voltage overshoot appears at the capacitor 5 which then leads to the ignition of the gas discharge lamp. In order to extend the lifetime of the gas discharge lamp 10 it is desirable to pre-heat the two lamp coils of the gas discharge lamp 10 before the ignition. For this purpose, there is provided a heating transformer having a primary winding 7A and secondary windings 7B and 7C, whereby the primary winding 7A is connected with the series resonance circuit whilst the secondary windings 7B and 7C are connected each in parallel to one of the lamp coils. By means of the connection of the secondary windings 7B and 7C to the lamp coils of the gas discharge lamp 10 it is possible to supply the lamp coils with energy also in ignited operation of the gas discharge lamp 10.
The central control unit shown in FIG. 2 serves for the generation of control signals for the operation of the gas discharge lamp 10, whereby the control signals of the central control unit 3 are generated in dependence upon internal and external operating state and control information. Thus, for example, the central control unit 3 monitors the heating current i.sub.H flowing via the primary winding 7A of the heating transformer in that the voltage dropped across a resistance 8 is delivered to the central control unit 3. Further, the central control unit 3 receives a parameter corresponding to the lamp current i.sub.L flowing through the gas discharge lamp path of the gas discharge lamp 10, in that a voltage dropped across a resistance 9 is delivered to the central control unit 3. As will be described in more detail below, it is also desirable to deliver to the central control unit 3 the impedance angle of the load circuit connected to the inverter 2. For this purpose, a phase comparator 12 determines the phase angle between the lamp voltage U.sub.L and the lamp current i.sub.L. Further, the lamp voltage U.sub.L is directly applied to the central control unit 3 itself as operating state information. As shown in FIG. 2, the central control unit 3 also monitors the mains voltage U.sub.N and the rectified intermediate circuit voltage U.sub.G delivered from the rectifier 1.
Along with these items of internal operating state information, the central control unit 3 however also evaluates external control information I.sub.extern which is delivered for example as desired-value information (e.g. for the dimming of the gas discharge lamp 10) or as illumination actual-value information of the central control unit 3, detected by a light sensor, via a (serial) interface.
It is known to provide the central control unit 3 in the form of a microcontroller, i.e. microprocessor, which centrally receives and evaluates all internal and external information and issues corresponding control signals for the operation of the gas discharge lamp 10. These control signals may for example switch the inverter 2 on or off or vary the frequency f or the duty ratio d of the a.c. voltage delivered by the inverter 2. Such a microcontroller, however, carries out its control functions, i.e. the generation of control signals, exclusively on the basis of a corresponding software programming, so that a central control unit 3 formed solely by means of a microcontroller is not suitable for rapid control processes, since before generation of a corresponding control signal the microcontroller must always first carry out the corresponding software program.
It is further known to realise the central control unit 3 exclusively by combining predetermined standard circuits, for example in the form of an integrated circuit. In this case, the central control unit 3 is provided exclusively as hardware. Such a central control unit 3 realized purely in hardware terms has however the disadvantage that the control circuit can be adapted only with difficulty to, for example, changes relating to the control or operating state information to be detected. That is, with a pure hardware realisation of the central control unit 3, flexibility is greatly restricted.
If the central control unit is provided as a software controlled microcontroller, the central control unit can be adapted by simple manner and means to circuit changes etc., i.e. flexibility is high, but on the other hand, as has already been described above, the central control unit is in this case not suitable for rapid procedures.
From U.S. Pat. No. 5,107,184 there is known a control circuit in accordance with the preamble of claim 1, which is employed in an electronic ballast for the operation of gas discharge lamps. Thereby, with the aid of an integrated circuit, a plurality of items of internal operating state information.
If the central control unit is provided as a software controlled microcontroller, the central control unit can be adapted by simple manner and means to circuit changes etc., i.e. flexibility is high, but on the other hand, as has already been described above, the central control unit is in this case not suitable for rapid procedures generates operational information for the second control unit. Primarily, however, the first control unit functions as a monitoring device or communications device. The second control unit is clearly controlled in terms of hardware. Implicitly, it appears that the known ballast must further contain memory means for storing operational information.
In accordance with EP-A-582 287 there is known a phase angle or phase segment dimmer having a first control unit controlled purely in terms of software and a second control unit controlled purely in terms of hardware. The first control unit, controlled in terms of software, receives external control information and generates operational information for the second control unit. Along with this, however, the control unit controlled purely in terms of software also deals with monitoring functions. The second control unit controlled in terms of hardware processes the operational state information. The known dimmer also has memory means for storing operational information.
In accordance with DE-U-29 610 289 there is known an electronic ballast having a first control unit and a second control unit. The first control units include a CPU, so it can be assumed that this first unit is controlled in terms of software. It receives external control information and generates operational information for the second control unit. Primarily, however, the first control unit functions as a monitoring device or communications device. The second control unit is clearly controlled in terms of hardware. Implicitly, it appears that the known ballast must further contain memory means for storing operational information.
In accordance with EP-A-582 287 there is known a phase angle or phase segment dimmer having a first control unit controlled purely in terms of software and a second control unit controlled purely in terms of hardware. The first control unit, controlled in terms of software, receives external control information and generates operational information for the second control unit. Along with this, however, the control unit controlled purely in terms of software also deals with monitoring functions. The second control unit controlled in terms of hardware processes the operational state information. The known dimmer also has memory means for storing operational information.
In accordance with U.S. Pat. No. 4,086,804 there is known a pneumatic pressure supply system which includes an external digitally functioning control loop and an internal analog functioning control loop.